In hydraulic systems, even with a fully filled fluid circuit, normal operation may create pressure disturbances within the fluid of sub second duration and/or frequency. Fluid disturbance effects may be considered as falling into two general categories. For example, in power steering hydraulic systems, a first fluid disturbance effect may be considered as relating to vehicle reliability insofar as disturbances may build to a resonance and/or impose stresses on components that could lead to premature component failure and/or loss of vehicle control. A second fluid disturbance effect may be considered as relating to nuisance insofar as the disturbance gives rise to noises and/or vibrations noticeable to the vehicle occupants. “Reliability” disturbances indiscernible by the occupants may thus not constitute a nuisance as such, just as “nuisance” disturbances may not affect the reliability of the vehicle. Notwithstanding the apparent inconsequential nature of “nuisance” disturbances, they are of significant importance to vehicle manufacturers insofar as they have profound effects on customer satisfaction with the product.
It will be appreciated that whether or not a physical disturbance effect is considered as noise or a vibration depends upon its frequency relationship to the physiologically defined senses of any occupant or user; in physical terms both are vibrations and are hereinafter considered as such except when specifically distinguished. Such variety of disturbances which affect, that is, are noticeable to, the occupants of motor vehicles are also often called by the collective term “noise, vibration, harshness” or its abbreviation “NVH”.
Moreover, these issues are not limited to a fluid actuation device in the form of a vehicle steering mechanism for which the fluid is a hydraulic liquid. Analogous fluid driven actuation devices for non vehicular use and/or devices using gaseous driving fluid may suffer some or all of the outlined effects and benefit from an alternative approach to damping.